(1) Field of the Invention
The present invention relates to a plastic-based multilayer structure, especially one suitable for the manufacture of tubes or pipes intended for transporting liquid fluids such as alcohols, petroleum-derived liquid fuels, fuels possibly containing oxygen compounds such as, for example, alcohols.
(2) Description of related Prior Art including information under 37 CFR 1.97 and 1.98
It relates more particularly to a multilayer structure having improved barrier properties, especially with respect to the abovementioned fluids.
The use of plastic tubes and pipes for transporting fluids such as fuels in internal-combustion engines is known. For example, nylon-12 tubes or pipes are often used in motor vehicles.
However, the standards relating to the construction of vehicles, especially those trying to combat the atmospheric pollution generated by vehicles, are becoming increasingly stringent.
Consequently, pipes and tubes having a monolayer structure, for example those made of nylon-12, cannot meet the new specifications imposed on internal-combustion engines. Thus, it is becoming very difficult to meet the required standards and characteristics regarding the fuel-vapour barrier property while still retaining suitable mechanical properties, such as flexibility, crack resistance and dimensional stability.
To remedy these drawbacks, tubes or pipes having a multilayer structure, comprising a polyamide, especially nylon-11 or nylon-12, layer and an ethylene/vinyl alcohol layer (see French Patent No. 2,579,290, EP 0,428,834 and EP 0,428,833). However, use of these tubes has demonstrated that they were unable to meet all the required characteristics for use in internal-combustion engines.
A multilayer structure has also been proposed which comprises a nylon-11 as the outer layer and a polyvinylidene fluoride as the inner layer. The polyvinylidene fluoride is plasticized, especially with a benzene sulphonamide. However, as in the previous case, such structures cannot meet the required characteristics for the main application of them, for example fuel pipes of internal-combustion engines. In particular, such a structure is defective with regard to adhesion between the layers.
Multilayer structures have also been proposed which comprise polyvinylidene fluoride layers bonded to polyamide layers. In order to improve the adhesion between these layers and therefore the properties of the entire multilayer structure, it has been proposed to use a polyvinylidene fluoride composition comprising either a glutarimide polymer (EP 0,637,511) or an acrylate copolymer comprising at least anhydride functional groups obtained by the cyclization of two adjacent carboxylic functional groups. However, the adhesion between the various layers of the structure and the mechanical properties of the latter deteriorate during aging.
One of the objects of the present invention is to propose a multilayer structure having improved mechanical and barrier properties and having a set of properties suitable for the manufacture or production of pipes or tubes for transporting fluids, especially fuels, more particularly fuels containing oxygenated compounds such as alcohol.